1. Field of the Invention
This invention generally relates to a laser medium and more particularly to a laser medium for use in a slab laser (hereunder referred to simply as a slab laser medium) which can weaken amplified spontaneous emission (hereinafter abbreviated as ASE) and suppress parasitic oscillation to thereby increase an oscillation efficiency or an amplification efficiency.
2. Description of the Related Art
As a conventional solid state laser medium, is publicly known a slab laser medium which has a slab structure provided with two parallel planes facing each other as reflecting inner surfaces (hereunder referred to simply as reflecting surfaces) as disclosed in, for example, Japanese Patent Application Publication No. 48-15599 Official Gazette. This conventional slab laser medium is used to perform laser oscillation or optical amplification by extracting a laser beam therefrom. Further, in this conventional slab laser medium, the laser beam follows a zigzag path undergoing internal reflection at the alternate reflecting surfaces. Therefore, even if the distance between the reflecting surfaces is short, the optical path followed by the laser beam can be sufficiently long. In other words, even if the laser medium is made thin, a desired path length can be obtained. Thereby, the laser medium can be efficiently cooled. Thus, large pump energy can be supplied to the laser medium. This realizes laser oscillation providing a large laser output.
Further, in general, where a thermal gradient is presented within a laser medium, thermal lensing and thermal birefringence occurring due to thermally induced distortion and stress cause phase differences among laser beams to be extracted. This results in degradation of beam quality. However, in case of this conventional slab laser medium, the laser beam goes along the zigzag path between the reflecting surfaces as described above. Thus, the laser beam equally and repeatedly travels obliquely to a transverse direction, in which the thermal gradient is presented, perpendicular to the two reflecting surfaces. Consequently, the phase difference due to unevenness of refractive index in the laser medium, which is caused by the thermal lensing and the thermal birefringence, is substantially cancelled, and further a laser beam with relatively good beam quality can be obtained.
As a conventional laser medium obtained by making better use of the characteristic of this slab laser medium to improve beam quality, is publicly known what is called a composite slab type laser medium proposed by J. L. Emmett et al (see The Potential of High-Average-Power Solid State Lasers UCRL-53571, Lawrence Livermore National Laboratory, California, 1984). This composite slab type laser medium includes a laser activating material only in a specific region between the reflecting surfaces to decrease the thermal gradient. Generally, in a slab laser medium, temperature is high in a central portion in the transverse direction between the two reflecting surfaces. Further, the closer to end portions (i.e., to the reflecting surfaces) a portion, the lower temperature. Thus, by removing the laser activating material from the central portion, generation of heat therein is prevented. Moreover, by making laser pumping regions of the end portions extremely thin, the thermal gradient in the transverse direction is made to be very small.
This slab laser medium, however, has encountered problems of the ASE and a parasitic oscillation caused in the inside thereof, which are obstacles to obtain a larger laser output. Incidentally, the ASE is an emitted light, which is stimulated and amplified by fluorescence in a laser medium and attenuates energy stored prior to normal laser oscillation and optical amplification. Further, the parasitic oscillation is a phenomenon that in a laser medium, a part of laser beams do not go along a normal optical path to be followed by a laser beam which resonates in the laser medium (hereunder sometimes referred to as a resonant optical path) but perform a harmful oscillation by, for instance, going back and forth many times between the reflecting surfaces in the transverse direction. Further, if the parasitic oscillation frequently occurs, the efficiency of effective laser oscillation and the efficiency of the amplification are decreased and as a result a large laser output cannot be obtained.
A known technique of suppressing this parasitic oscillation is what is called a segmented spacer (see "New Slab and Solid-State Laser Technology and Application", SPIE., Vol. 736, p. 38, 1987). According to this technique, a gasket member made of rubber and so on is put into contact with an outer surface of each of parts, at which a laser beam is not reflected, of the parallel planes in order to prevent conditions of total internal reflection from holding. As described above, in the slab laser medium, a laser beam to be extracted therefrom (hereunder sometimes referred to simply as an extraction beam) goes along a zigzag path undergoing reflection at the alternate reflecting surfaces. As a consequence, each reflecting surface is scattered with parts of a region (hereinafter referred to as a non-path region), through which the extraction beam does not pass. Therefore, the efficiency of oscillation is not decreased in case where the conditions of total reflection of the laser beam are made not to hold for parts of the non-path region. Moreover, by preventing the conditions of total reflection from holding for parts of the non-path region, reflection of light generated by the ASE or the parasitic oscillation having reached the parts of the non-path region can be prevented, Furthermore, attenuation of the stored energy can be suppressed.
The segmented spacer is developed on the basis of an idea that reflection of a laser beam at parts of the non-path region is restrained by making the conditions of total reflection from holding for the parts of the non-path region. Thus, the gasket member is used as a member for making the conditions of total reflection from holding.
However, the results of the experiments made by inventors of the present invention reveals that the gasket member is very easily deteriorated by iteration of the laser oscillation and optical amplification. From an investigation, it is found that the cause of this is a phenomenon that the gasket member is not also heated by heat conducted from the laser medium but also absorbs pumping light and light emitted due to parasitic oscillation (hereinafter referred to as parasitic oscillation light) and generates heat and thus temperature of the gasket member is liable to rise to a permissible temperature and higher. Especially, this phenomenon is conspicuously presented in case that an air-cooling method with low cooling efficiency is employed for cooling the laser medium.
The present invention is intended to obviate the above described drawbacks of the conventional slab laser.
It is accordingly an object of the present invention to provide a slab laser which can effectively suppress parasitic oscillation and stably perform laser oscillation and light amplification for a long period of time.